Radio links are widely used in telecommunications, e.g. micro wave links operating at 1 to 20 GHz, and a linkage normally comprises a transmitter beamed to a receiver. It is the intention of the designer that the signals shall travel in a single line-of-sight path but, under certain conditions, reflection or diffraction may occur whereby the receiver obtains signals via more than one path. Since the paths are of different lengths the signals from the longer path are delayed and hence there are frequency dependent phase differences between the received signals. Under adverse conditions, these can cause severe distortion, e.g. notches in the frequency domain, and it is conventional to include in the receiver stages which are designed to reduce this effect.
In order to test these stages it is necessary to utilise test equipment which simulates the distortion so that their performance can be checked. The test equipment is inserted into the circuitry of a radio to be tested upstream of the correction stages, e.g. immediately after the frequency change for radios in which correction is applied during the IF stages or at base band. In order to check performance over a range of conditions it is necessary to adjust the test equipment to provide a variety of simulated interference conditions and, in particular, it is desirable to simulate 99 percent amplitude cancellation leaving a residue of about 1 percent. Since this is produced by a discrepancy between two almost equal paths it would be necessary to use very accurate components and components of sufficient accuracy are either not available at all or too costly for use in commercial equipment. It is an object of this invention to obtain the delay without the need for accurate components.